The classical method for converting fuel energy into heat is the oxidation of the fuel into flames with an oxidizing agent. When natural substances (coal or hydrocarbons with air) are combusted, the flame temperature is in the range from approximately 1600.degree. to 2000.degree. C., with the advantage that after the ignition event the oxidation of the fuel proceeds on its own in the flame front, even under unfavorable conditions (such as an open fire in a fireplace, or the like). This principle is also adhered to when gaseous or liquid fuels are combusted with burners; provisions for stabilizing the flame play a major role. Burners with a capacity ranging from a few kW to several MW are classified, depending on the delivery of fuel and air, as premix burners and orifice mix burners. In the case of the latter type, the combustion air can be preheated up to very high temperatures in order to recover the exhaust gas heat. The ratio between the preheated temperature of the combustion air and the temperature of the exhaust gas upon leaving the combustion chamber (both expressed in .degree. C.) is known in somewhat simplified terms as the relative air preheating .epsilon.. In the industry, air preheating values .epsilon. of approximately 0.7 are achieved in so-called recuperator burners, and up to approximately 0.9 with so-called regenerator burners.
Preheating of the combustion air does allow considerable economy in energy; however, it increases the flame temperature and thus leads to a very major increase in thermal NOx formation. The symbol NOx designates nitrogen oxides in general. These nitrogen oxides are undesirable because of the environmental pollution they cause. Legal regulations accordingly exist that limit the allowed NOx emissions, and the trend is to increasingly lower the allowable limits.
Known low-NOx burners operate with so-called graduated combustion (German Pat. No. 34 22 229) and/or cooling of the flame with recirculated exhaust gas (see, for instance, Gaswarme International [Gas Heat International], Vol. 38 (1939), issue 5, June/July, pp. 283-292; page 290).
To cool the flame and lower NOx emissions, exhaust gas from the combustion chamber is either re-aspirated hydraulically (internal recirculation) or resupplied from outside (external recirculation). For the sake of flame stability, the admixture of exhaust gas is limited to a maximum of approximately 50% referred to the quantity of combustion air.
For flame stabilization it is additionally known to return hot gas directly from the flame to the flame root, and with burners that use free-flowing fuels, to return this hot exhaust gas first to the fuel stream, in order to evaporate the fuel, as a rule oil, prior to combustion and admixture of the combustion air (these are known as "blue burners"). One example of this is described in German Pat. No. 23 03 280.
In addition to combusting fuel in flames, so-called flameless combustion of the fuel is also known, which has already been industrially employed in radiant burners (Gaswarme lnternational, Vol. 34, (1985), issue 4, April, pp. 162-166). The oxidation of the gaseous fuel occurs directly at the surface of a porous, usually ceramic body, which gives off the heat by radiation. The relatively low temperature of the oxidation leads to low NOx formation. Since the fuel is premixed with the combustion air, however, the combustion air can be preheated only a little, because otherwise premature ignition or thermal decomposition of the fuel will occur. This makes these burners uneconomical for relatively high capacities and high temperatures (above approximately 900.degree. C.); moreover, the shape of the radiant body places limits on their use.